The present invention relates to well logging, and in particular to a new method and apparatus for acoustically imaging the wall of a bore-hole in fine detail to determine the microstructure of the formations penetrated by the borehole. The invention, which utilizes some acoustical imaging techniques similar to those used with borehole televiewers (BHTV's), is distinct from borehole televiewers in terms of the type of image and detail generated and the precise means by which it is done. However, the invention is particularly useful in conjunction with tools such as borehole televiewers, as will be explained further below.
In this regard, reference may be made to several patents relating to borehole televiewers, such as U.S. Pat. Nos. 3,369,626 (Zemanek, Jr., issued Feb. 20, 1968), 3,478,839 (Zemanek, Jr., issued Nov. 18, 1969), 4,463,378 (Rambow, issued July 31, 1984), and 4,601,024 (Broding, issued July 15, 1986). In general, such borehole televiewer logging tools operate acoustically by periodically pulsing a rotating acoustic transducer to emit a sequence of acoustical pulses directionally into the borehole toward the borehole wall, and analyzing the echoes which are reflected back to the tool. The amplitude of the reflected signal may then be displayed on a cathode ray tube, the display sometimes being photographed for future reference. Typically, the display represents a map of the borehole wall split along the north direction and laid out flat. Alternately, a polar display may be produced, in which case the radius of the circular trace is determined by the time-of-flight of the acoustic pulse, thus presenting a cross-sectional profile of the borehole. Another display, similar to the amplitude display, is modulated by the time-of-flight signal rather than the amplitude signal. The latter can be converted into a pseudo-three-dimensional image by adding a slight bias to the vertical sweep according to the magnitude of the time-of-flight signal.
The higher the resolution, of course, the more valuable will be the information reported by BHTV logging tools. In the recent past, substantial improvements have been made in the physical design and choice of materials for ever better resolution of the images. As will become apparent, however, due to the competing objectives there will probably always be certain compromises forced upon borehole televiewer type logging tools. That is, a primary objective is to provide a complete "visual" image. This requires an essentially continuous scan around the circumference of the borehole wall. Thus, to scan or read so many contiguous target elements, the solution adopted has been to locate the tool centrally within the borehole and scan a beam radially around the borehole, bouncing it off the target points on the borehole wall back to the borehole televiewer. To physically contact and read the same number of points would appear to be highly impractical.
Thus, since the acoustical transducer is accordingly located some distance away from its target, a certain loss of resolution is to be expected. First, from the very nature of the transducer, it can be expected that the target size or footprint of the transmitted acoustical beam, at the borehole wall, will expand somewhat from its size nearer to the transducer (depending, of course, on the particular characteristics at hand). A certain scattering of the acoustical energy will also take place as it propagates through the various media in the BHTV and within the borehole. Additionally, since the borehole wall is rarely smooth and flat, the beam will be somewhat scattered or jumbled by different reflecting surfaces at different angles within the area of the footprint on the borehole wall. This effect will be effectively increased generally in relation to the distance of the target area on the borehole wall from the acoustical transducer in the borehole televiewer. Accordingly, thinly laminated formation beds (e.g., less than 1/3 inch), often seen, for example, in certain depositional environments, may not be well resolved with such tools. The problems can be exaggerated in heavy mud weights, large diameter boreholes, or in wash-out zones within the well.